Legacy effects of eutrophication on modern methane dynamics in a boreal estuary

Estuaries are important conduits between terrestrial and marine aquatic systems and function as hot spots in the aquatic methane cycle. Eutrophication and climate change may accelerate methane emissions from estuaries, causing positive feedbacks with global warming. Boreal regions will warm rapidly in the coming decades, increasing the need to understand methane cycling in these systems. In this 3-year study, we investigated seasonal and spatial variability of methane dynamics in a eutrophied boreal estuary, both in the water column and underlying sediments. The estuary and the connected archipelago were consistently a source of methane to the atmosphere, although the origin of emitted methane varied with distance offshore. In the estuary, the river was the primary source of atmospheric methane. In contrast, in the adjacent archipelago, sedimentary methanogenesis fueled by eutrophication over previous decades was the main source. Methane emissions to the atmosphere from the study area were highly variable and dependent on local hydrodynamics and environmental conditions. Despite evidence of highly active methanogenesis in the studied sediments, the vast majority of the upwards diffusive flux of methane was removed before it could escape to the atmosphere, indicating that oxidative filters are presently still functioning regardless of previous eutrophication and ongoing climate change.Peer reviewe

Autochthonous organic matter promotes DNRA and suppresses N2O production in sediments of the coastal Baltic Sea

Coastal environments are nitrogen (N) removal hot spots, which regulate the amount of land-derived N reaching the open sea. However, mixing between freshwater and seawater creates gradients of inorganic N and bioavailable organic matter, which affect N cycling. In this study, we compare nitrate reduction processes between estuary and offshore archipelago environments in the coastal Baltic Sea. Denitrification rates were similar in both environments, despite lower nitrate and carbon concentrations in the offshore archipelago. However, DNRA (dissimilatory nitrate reduction to ammonium) rates were higher at the offshore archipelago stations, with a higher proportion of autochthonous carbon. The production rate and concentrations of the greenhouse gas nitrous oxide (N2O) were higher in the estuary, where nitrate concentrations and allochthonous carbon inputs are higher. These results indicate that the ratio between nitrate and autochthonous organic carbon governs the balance between N-removing denitrification and N-recycling DNRA, as well as the end-product of denitrification. As a result, a significant amount of the N removed in the estuary is released as N2O, while the offshore archipelago areas are characterized by efficient internal recycling of N. Our results challenge the current understanding of the role of these regions as filters of land-to-sea transfer of N.Peer reviewe

The analysis of freshwater pearl mussel shells using µ-XRF (Micro X-Ray Fluorescence) and the applicability for environmental reconstruction

Peer reviewe

Depth and intensity of the sulfate-methane transition zone control sedimentary molybdenum and uranium sequestration in a eutrophic low-salinity setting

Molybdenum (Mo) and uranium (U) contents in sedimentary archives are often used to reconstruct past changes in seafloor oxygenation. However, their sequestration processes are as yet poorly constrained in low-salinity coastal waters, which often suffer from anthropogenic eutrophication but only mild oxygen depletion. Due to the consequent lack of robust long-term paleo-redox reconstructions in such settings often characterized by a shallow front of dissolved sulfide accumulation within the sediment pore waters, inadequate understanding of the long-term drivers behind oxygen loss impedes cost-effective mitigation of this environmental problem. Here, we investigate the mechanisms of Mo and U sequestration in an oxic, low-salinity coastal setting in the northern Baltic Sea where anthropogenic eutrophication over the 20th century has resulted in formation of a shallow sulfate-methane transition zone (SMTZ) in the sediment column of this brackish-water basin. Our results demonstrate remarkably similar patterns for authigenic Mo and U sequestration, whereby the depth and intensity of the SMTZ exerts a first-order control on their solid-phase uptake. Sequential extraction analysis suggests that a large part of the authigenic Mo pool is hosted by refractory Fe-S phases such as pyrite and nanoscale FeMoS4, implying that the Fe-sulfide pathway is the dominating process of authigenic Mo scavenging. However, we also observe a pool of extremely labile Mo deep within the SMTZ, which might record an intermediate phase in authigenic Mo sequestration and/or partial switch to the organic matter (OM) pathway at low dissolved Fe levels. Authigenic U resides in acid-extractable and refractory phases, likely reflecting uptake into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Similarly to Mo, authigenic U uptake is active at two fronts within the SMTZ, paralleled by increases in dissolved sulfide levels, suggesting coupling between sulfide production and U reduction. Our results imply that both Mo and U could provide viable proxies for mild bottom water deoxygenation in these settings, through the indirect link between seafloor oxygen conditions and the depth of SMTZ. Of these, Mo appears to more robustly capture variations in seafloor oxygen levels due to the significantly higher share of the authigenic pool. However, temporal resolution of these proxies is limited by the vertical offset between seafloor and the zone of authigenic uptake, and the superimposed character of the signal at a given depth due to vertical migrations of the SMTZ. These results have important implications for the use of Mo and U as paleo-redox proxies in other low-salinity coastal settings exposed to eutrophication.Peer reviewe

Terrestrial organic matter input drives sedimentary trace metal sequestration in a human-impacted boreal estuary

Coastal sediments play a fundamental role in processing anthropogenic trace metal inputs. Previous studies have shown that terrestrial organic matter (OM) is a significant vector for trace metal transport across the land-to-sea continuum, but little is known about the fate of land-derived metal-OM complexes in coastal sediments. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to investigate how variations in terrestrial OM delivery since the 1950s have influenced trace metal accumulation and diagenesis in a human-impacted boreal estuary in the northern Baltic Sea. A key feature of our dataset is a strong correlation between terrestrial OM deposition and accumulation of metal-OM complexes in the sediments. Based on this strong coupling, we infer that the riverine input of terrestrial metal-OM complexes from the hinterland, followed by flocculation-induced settling in the estuary, effectively modulates sedimentary trace metal sequestration. While part of the trace metal pool associated with these complexes is efficiently recycled in the surface sediments during diagenesis, a substantial fraction is permanently buried as refractory metal-OM complexes or through incorporation into insoluble sulfides, thereby escaping further biological processing. These findings suggest that terrestrial OM input could play a more pivotal role in trace metal processing in coastal environments than hitherto acknowledged. (c) 2020 The Authors. Published by Elsevier B.V.Peer reviewe

Hypoxia in the Holocene Baltic Sea : Comparing modern versus past intervals using sedimentary trace metals

Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8-4 ka before present; BP) and the Medieval Climate Anomaly (MCA(HI); similar to 1200-750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with C-org and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (Modern(HI)) comparatively less Mo is sequestered relative to C-org than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the Modern(HI) is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCA(HI) and Modern(HI), possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between C-org and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the C-org flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution.Peer reviewe

Role of particle dynamics in processing of terrestrial nitrogen and phosphorus in the estuarine mixing zone

Multiple biogeochemical processes in estuaries modulate the flux of nutrients from land to sea, thus contributing to the coastal filter. The role of particle dynamics in regulating the fate of terrestrial nutrients in estuaries is poorly constrained. To address this issue, we resolved the particle size distribution of suspended material, and quantified size-fractionated particulate nitrogen (PN) and phosphorus (PP), in a stratified mesotrophic estuary (Pojoviken, Finland). We also carried out a mixing experiment where the effects of salt-induced flocculation on particle size distribution and concentrations of PN and PP were examined. The experimental results showed that salt-induced flocculation at already very low salinities increases the total particle concentration and mean particle size, indicating transfer of dissolved material into particulates. Correspondingly, a significant increase in PP and particulate iron (Fe) was observed in the experiment results, suggesting coupled flocculation of P-containing organic matter (OM) and ferrihydrite. Particle dynamics in the field data were dominated by processes occurring downstream of the flocculation zone. Primary production created a downward flux of autochthonous OM particles, promoting passive aggregation by random collisions with terrestrial material in the water column. Maximum particle concentrations were observed at and below the halocline. The highest PN and PP concentrations were observed in the subhalocline layer, 3.5 and 0.14 mu mol L-1, respectively. Molar ratios of N:P in this material were >40, consistent with typical marine snow in the early stages of microbial processing. Our study provides a mechanistic overview of the biogeochemical drivers of particulate nutrient dynamics in stratified estuarine environments.Peer reviewe

16S and 18S rRNA Gene Metabarcoding Provide Congruent Information on the Responses of Sediment Communities to Eutrophication

Metabarcoding analyses of bacterial and eukaryotic communities have been proposed as efficient tools for environmental impact assessment. It has been unclear, however, to which extent these analyses can provide similar or differing information on the ecological status of the environment. Here, we used 16S and 18S rRNA gene metabarcoding to compare eutrophication-induced shifts in sediment bacterial and eukaryotic community structure in relation to a range of porewater, sediment and bottom-water geochemical variables, using data obtained from six stations near a former rainbow trout farm in the Archipelago Sea (Baltic Sea). Shifts in the structure of both community types were correlated with a shared set of variables, including porewater ammonium concentrations and the sediment depth-integrated oxygen consumption rate. Distance-based redundancy analyses showed that variables typically employed in impact assessments, such as bottom water nutrient concentrations, explained less of the variance in community structure than alternative variables (e.g., porewater NH4+ inventories and sediment depth-integrated O2 consumption rates) selected due to their low collinearity (up to 40 vs. 58% of the variance explained, respectively). In monitoring surveys where analyses of both bacterial and eukaryotic communities may be impossible, either 16S or 18S rRNA gene metabarcoding can serve as reliable indicators of wider ecological impacts of eutrophication.Peer reviewe

Із зали засідань Президії НАН України (24 жовт­ня 2012 року)

На черговому засіданні Президії НАН України 24 жовтня 2012 року члени Президії НАН України та запрошені заслухали такі питання: про діяльність Державного агентства з питань науки, інновацій та інформатизації України з удосконалення нормативно-правової бази у сфері наукової, науково-технічної, інноваційної діяльності та інформатизації протягом 2011–2012 рр. (доповідач — голова Агентства академік НАН України В.П. Семиноженко); про наукову та науково-організаційну діяльність Інституту хімії високомолекулярних сполук НАН України (доповідач — академік НАН України Є.В. Лебедєв); сучасні уявлення про механізми тертя (доповідач — доктор фізико-математичних наук О.М. Браун); про нагородження відзнаками НАН України та Почесними грамотами НАН України і Центрального комітету профспілки працівників НАН України (доповідач — академік НАН України В.Ф. Мачулін); кадрові та поточні питання

Drought recorded by Ba/Ca in coastal benthic foraminifera

Increasing occurrences of extreme weather events, such as the 2018 drought over northern Europe, are a concerning issue under global climate change. High-resolution archives of natural hydroclimate proxies, such as rapidly accumulating sediments containing biogenic carbonates, offer the potential to investigate the frequency and mechanisms of such events in the past. Droughts alter the barium (Ba) concentration of near-continent seawater through the reduction in Ba input from terrestrial runoff, which in turn may be recorded as changes in the chemical composition (Ba/Ca) of foraminiferal calcium carbonates accumulating in sediments. However, so far the use of Ba/Ca as a discharge indicator has been restricted to planktonic foraminifera, despite the high relative abundance of benthic species in coastal, shallow-water sites. Moreover, benthic foraminiferal Ba/Ca has mainly been used in openocean records as a proxy for paleo-productivity. Here we report on a new geochemical data set measured from living (CTG-labeled) benthic foraminiferal species to investigate the capability of benthic Ba/Ca to record changes in river runoff over a gradient of contrasting hydroclimatic conditions. Individual foraminifera (Bulimina marginata, Non-ionellina labradorica) were analyzed by laser-ablation ICP-MS over a seasonal and spatial gradient within Gullmar Fjord, Swedish west coast, during 2018-2019. The results are compared to an extensive meteorological and hydrological data set, as well as sediment and pore-water geochemistry. Benthic foraminiferal Ba/Ca correlates significantly to riverine runoff; however, the signals contain both spatial trends with distance to Ba source and species-specific influences such as micro-habitat preferences. We deduce that shallow-infaunal foraminifera are especially suitable as proxy for terrestrial Ba input and discuss the potential influence of water-column and pore-water Ba cycling. While distance to Ba source, water depth, pore-water geochemistry, and species-specific effects need to be considered in interpreting the data, our results demonstrate confidence in the use of Ba/Ca of benthic foraminifera from near-continent records as a proxy for past riverine discharge and to identify periods of drought.Peer reviewe